Tuning apparatus for high frequency receiving systems



Qct. 18, 1938. c. sun's I 2,133,878

TUNING APPARATUS FOR HIGH FREQUENCY RECEIVING SYSTEMS Filed June 28, 1955 TON/N6 V0/ A R All IA v a a "0 NW. 0'0 {60111 I w Invent. or:

Mm Chauncgg G. Suit; y WM 5.

His Attorney Patented Oct. 18, 1938 PATENT OFFICE TUNING APPARATUS FOR HIGH FREQUENCY RECEIVING SYSTEMS Chauncey G. Suits, Schenectady, N. Y., assignor to General Electric Company, a

New York corporation of Application Jane 28, 1935, Serial No. 28,822

12 Claims.

My invention relates to tuning means for radio receiving systems, and more particularly to a method of and means for automatically tuning a radio receiving system to exact resonance with 6 a selected carrier of a particular frequency following a rough manual adjustment of the system to respond approximately to the selected carrier frequency.

My invention is particularly applicable to receiving systems of the superheterodyne type which include an automatic volume control circuit, since by its use an exact and accurate adjustment of the tuning elements may be obtained automatically thereby eliminating the dependence of the operator upon the sense of sight or hearing for securing the correct tuning adjustment. Since failure to tune a receiving system to an exact resonant condition at the selected carrier frequency results in inefficient operation got and a sacrifice in the quality of reception, it is obviously advantageous to make the accurate tuning independent of the operators skill. In accordance with my invention, the accurate tuning is performed automatically following a rough 25-. manual adjustment of the system to resonate with the carrier which it is desired to receive.

In this manner but little skill on the part of the operator is required to obtain the maximum efficiency and quality of reception from the set.

It is an object of my invention to provide a method of, and means for, automatically tuning a radio receiving system to exact resonance with a selected carrier frequency in response to a manual adjustment of the system to an approximate resonant condition at the selected carrier frequency.

More specifically it is an object of my invention to provide in a radio receiving system of the superheterodyne type an automatic tuning device whereby the system may be tuned exactly to a selected carrier frequency; the automatic tuning device being controlled in accordance with the difference between the selected operating intermediate frequency and the frequency output of an intermediate frequency amplifier.

A further object of my invention is to provide an automatic tuning arrangement of the above type which comprises an actuating device arranged to be mechanically coupled to the gang controlled tuning elements and energized in accordance with the differential of current flowing in a network which is energized in accordance with the rectified currents flowing from a pair of resonating circuits electrically coupled to an 56, intermediate frequency channel of the system and tuned respectively above and below the selected operating intermediate frequency of the system.

The novel features which I believe to be characteristic of my invention are set forth with '5 particularity in the appended claims. My invention itself, however, both as to its organization and the method of operation will best be understood by reference to the following speci fication taken in connection with the accoml0 panying drawing in which the single figure illustrates a radio receiving circuit of the superhet-- erodyne type having my invention embodied therein.

Referring to the drawing I have shown a con- 15 ventional superheterodyne radio receiving circuit equipped with an automatic tuning device and control circuit therefor, arranged in accordance with my invention. The circuit includes a. radio frequency amplifier l coupled to an antenna sys- 2.0 tern 2 by the coupling transformer 3. The tunable signal input circuit formed by the secondary of the transformer 3 and a radio frequency bypass condenser 4 connected between the control grid and cathode of the amplifier tube, is tuned by a tunable element in the form of a variable condenser 5 shunted by a small trimmer condenser 6. The output from the amplifier l is impressed on the primary of a coupling transformer I having its secondary connected between the control grid and cathode of the first detector, or mixer tube 8. A radio frequency by-pass condenser 9 and the secondary of a coupling transformer l0 arranged to couple the local oscillator to the tube 8, are included in the in- 5 put circuit of the first detector A main tuning condenser H and a small trimmer condenser l2 are connected in parallel with the secondary of the coupling device 1 for tuning the input circuit of the detector to resonate with selected carrier current frequencies.

The local oscillator includes a triode l3 having its input and output circuits coupled through the condenser 14. The frequency determining circuit is comprised by the primary of the coua5 pling transformer ID, the main tuning condenser 15 (which constitutes the oscillator variable tuning element) and the parallel-connected trimmer condenser IS. A resistance l1 and a condenser is are provided for biasing the grid of tube l3 5() negative with respect to the cathode thereof. The output from the oscillator is impressed on the input circuit of the first detector 8 through the coupling device l0 where it mixes with the incom g radio frequency carrier current to pro- Q5 tube.

duce modulated intermediate frequency beat notes in a manner well known in the art.

The output circuit of the detector 8 includes the primary of a coupling transformer I9 having its secondary connected across the control grid and cathode of an intermediate frequency amplifier 20, through an intermediate frequency by-pass condenser 2| and an adjustable grid biasing resistance 22', the latter element being shunted by an intermediate frequency by-pass condenser 23. The output circuit of the detector 8 is tuned to the operating intermediate frequency by a condenser 24 and the input circuit of the amplifier 20 is tuned to the same frequency by a condenser 25.

The output from the intermediate frequency amplifier 20 is impressed on a second detector 26 by means of a coupling transformer 21. The transformer 2'! consists of a primary split into two series-connected sections 28 and 29 each tuned to the operating intermediate frequency by fixed condensers 30 and 3|. In like manner the secondary winding of transformer 21 is divided into two series connected sections 32 and 33, coupled respectively to the primary sections 28 and 29 and tuned to the operating intermediate frequency by fixed condensers 34 and 35. The two banks of coupled sections are shielded from each other to prevent electrostatic and electromagnetic coupling by shielding containers indicated diagrammatically at 36 and 31. The purpose of sectionalizing the coupling transformer, and shielding the coupled sections is explained in detail hereinafter.

The second detector 26 is provided in its input circuit with a resistance 38 and a condenser 39 for biasing the grid of the tube negative with respect to the cathode thereof. The detector 26 is resistance coupled to a driver stage which includes the tube 40 by means of the resistance 4| connected in the output circuit of the detector and shunted by the series-connected audio bypass condenser 42 and volume control potentiometer 43. A filter network is included in the output circuit of the detector 26 ahead of the coupling resistance 4|; the network comprising an intermediate frequency choke coil 44 in series with the plate circuit of tube 26 and a pairof intermediate frequency grounding condensers 45 and 46. A volume-frequency distortion compensating impedance is connected across a portion of the volume control potentiometer 43 and serves to compensate for frequency distortion at low volume intensities. This impedance consists of a resistance 41, inductance 48, and condenser 49 all connected in series between a tapped point on the potentiometer 43 and ground. A suitable bias is provided for the control grid of the driver tube 40 by the shunt-connected resistance 50 and condenser 5| connected in the cathode lead of the The driver stage output is impressed on the input of a power amplifier indicated diagrammatically at 52 by a coupling transformer 53 and the power amplifier output is in turn delivered to the loud speaker units 54 and 55 by a coupling transformer 56.

A rectifying system for supplying operating voltages to the above-described. receiver circuit is shown in the lower left portion of the drawing. This system includes the power transformer 51 having a low voltage secondary winding 58, and a high voltage winding 59 connectedto a full wave rectifier 60 which delivers direct current to the plate circuits of the several tubes through the filter network 6 I. The loudspeaker field coils 62 and 63 for speakers 54 and 55 respectively are connected in the output circuit of the rectifier.

Each of the tubes I, 8 and 20 is equipped with a screen grid for reducing inter-electrode capacity coupling between the input and output circuits of the tubes; a reduced positive biasing voltage being supplied to the two grids from a common screen grid circuit conductor which istapped at a suitable point on the speaker field 62. A tuning indicator 64 is connected in the anode circuit of the first detector tube and operates to give a visual indication of the sharpness with which the system is tuned to a selected carrier frequency. The indicator is of the shadowgraph type of which there are several well-known examples in the art.

An automatic volume control circuit is provided for supplying a direct current biasing potential varying in accordance with the intensity of an incoming carrier to the control grids of tubes I, 8 and 20. The volume control circuit comprises the tube 65 which is controlled by the intermediate frequency potential existing across resistance 66, the left portion of resistance 61 and condenser 10 connected in series across the intermediate frequency channel of the system through the intermediate frequency coupling condenser 69. The tube 65 is normally biased to cut-off by the potential across resistance 61. The alternating potential existing across resistance 66 and the left portion of resistance 61, is rectified by the discharge device 65 thereby producing a unidirectional voltage on resistance 1| having a magnitude which varies with the intensity of the received carrier. Desired smoothing of this voltage is effected by condenser 72. The negative terminal of this resistance is connected to the control grids of tubes l, 8 and 20 through resistances 14, I5 and 16.

The operation of the above-described system is well known in the art and only a brief description is deemed necessary to an understanding thereof. When an incoming modulated carrier is received on the antenna system 2 it is amplified by the radio frequency amplifier l and heterodyned with the locally generated oscillations from the oscillator l3. The mixed currents are detected in the first detector 8 and the modulated intermediate frequency output currents are then amplified in the amplifier 20, demodulated in the second detector 26, and supplied to the driver stage 40. The controlled driver supplies control current to the power amplifier 52 which drives the loudspeaker units 54 and 55.

Adjustment of the system to resonate at different received carrier frequencies is effected by the condensers 5, II and I5 which are mechanically connected to be controlled by a single control or unicontrol member indicated diagrammatically at 11. A manually operable element 18 is arranged to control the movement of the control member through a suitable mechanical coupling indicated diagrammatically by the rack 19 and pinion 80. It will be obvious that the mechanical connection between the manual element 18 and the tuning elements 5, I I and I5 insures an immediate adjustment of the receiver to respond to a new carrier frequency upon any slight adjustment of the element 18. The manual adjustment of the receiver willin practice result generally in a comparatively rough adjustment of the tuning elements 5 and H to a frequency approximately that of a desired signal carrier frequency.

In accordance with my invention the accurate tuning is accomplished automatically in response to a rough manual adjustment'of .the system tuning. To obtain such automatic tuning I provide a motor 8| which is arranged to actuate the condenser control member 11 through the reduction gearing 82 and a friction slip clutch 83. The motor may be of any type or construction suitable for operation in either direction under the influence of a pair of field coils. -I have'shown the motor 8| as being of the induction disk type polarized by a field coil84 which isenergized from a source of alternating potential 85. Control field coils 86 and 81 are included inthe motor structure and are arranged to give selective rotation in either direction depending upon the rela= tive degrees of energization of the two coils. The friction slipclutch 83 insures that the adjustment of the tuning elements 5, H and I5 shall always be under the immediate control of the manual element 18 regardless of any torque which the motor 8| may exert on the pinion in an effort to adjust the tuning elements to a higher or a lower frequency setting.

For controlling the degree and direction of energization of the field coils I provide a control network which includes a pair of resonating circuits 88 and 89 respectively coupled to the primary sections 28 and 29 of the intermediate frequency coupling transformer 21 and tuned-respectively above and below the operating intermediate frequency. The two resonating circuits 88 and 89 are connected respectively to apair of rectifying tubes 90 and 9|. An adjustable impedance consisting of an intermediate frequency by-pass condenser 92 and an adjustable biasing resistance 93 is provided for biasing the grids of the tubes 98 and 9| with respect to their respective cathodes. The output circuits from the two tubes 99 and 9l include the direct current windings 94 and 95 of a pair of saturable core reactors 96 and 91. The alternatingcurrent coils of the reactors 98 and 91 are respectively connected in series with the control coils 81 and 86 and across a suitable source of alternating potential 98. Smoothing condensers 99 and H10 are connected respectively across the direct current windings of the reactors 96 and 91 for smoothing out the direct current impulses generated by the tubes 90 and 9!. Normally the tubes 98 and 9| are biased to cut off by the potential across the potentiometer resistance 93. However, .with an alternatin potential applied to the grids, pulsating currents are supplied to the windings 94 and 95 which control the saturation of the'reactors 96 and 91. v r

The saturable core reactors 98 and 91 are well known in the art and may be of any standard commercial type having the correct impedance characteristics. Briefly, the reactors each comprise an alternating current winding, a direct current control winding and a saturable magnetic core. The operating characteristics are such that as the direct current flowing through the .control winding is increased, the core saturates, thereby decreasing the impedance of the alternating current winding. The pair of reactors 96 and 91 are alike in construction and possess identical operating characteristics. 2

The resonating circuits 88 and 89 are tuned to natural frequencies differing only slightly in either direction from the operating intermediate frequency. In a specific installation the circuit 88 was tuned to a frequency of 177 kilocycles, two kilocycles higher than the natural frequency of the circuit including the primary section 28, and the circuit 89 was tuned to a frequency of 1'73 kilocycles, two kilocycles lower than the natural frequency of the circuit including the primary section 29.

The tuning operation with the automatic tuning arrangement described is set forth below. With the system tuned off-resonance with any received carrier the current flowing inthe resonant circuits 88 and 89 is negligible'and no current flowsthrough the direct current windings of the reactors 96 and 91. It follows that the cur-. rents in the two control windings 85 and 81 are substantially zero, since the reactors are unsaturated and their alternating current impedance is high and, therefore, no torque is developed in the motor 8|. However, as the condensers 5, H and 15 are .manually operated to tune the system to approximate resonance with a selected received carrier, the frequency of the current in the primary, sections 28 and 29 of the transformer 21 approaches the .normal operating intermediate frequency. The direction of approach is obvious- 1y from a frequency value above the operating intermediate frequency, downward, or from a value'below the operating intermediate frequency upward, depending upon the direction of the manual tuning of the system to resonate with a selected, carrier frequency. Assuming the approach direction is from a frequency value below the operating intermediate frequency upward, a point is reached wherethecircuit 89 is energized by currentsoifa frequency equal to the, natural frequency of the circuit. Theoscillation of the currentsinthe circuit 89 causes the tube 9| to pass direct current tothe winding 95 of the saturable core, reactor 91, thereby to decrease the impedancegof the j, circuit including .the control winding 88 and increase the current flowing therethrough. The increase in current in the winding 86 causes adifferential in the magnitude of the currentsin the field coils 89 and 81 thereby developing torque in the motor 8| which is transmitted throughthe reduction gearing 82 and. the friction slip clutch 83 to. the condenser control member". Rotation of the condensers 5, H and I5 in response to the torque developed inthe motor 8| tends to increase the intermediate frequency since movement of thev condensers under the influence of the motor armature, decreases the frequency of the local oscillator output currents, thereby increasing the beat frequency between the incoming carrier frequency and the oscillator output frequency. This increase in the intermediate frequency continues under the infiuence of continued rotation of the motor armature until the currents flowing in the two motor control windings are equal in magnitude and opposite in phase. The condition of equal currents is realized when the intermediate frequency has attained its normal operating value midway be tween the natural frequencies of the two resonating circuits 88 and 89. g

If the system is being tuned in the opposite direction such that the intermediate frequency is decreased to its normal operating value, the same operating steps are carried out with the exception that the current in the control winding 81 is predominant, thereby causing rotation of the motor armature in a direction opposite to that noted above, whereby the intermediate frequencyis decreased until the current flowing in the windings 88 and 81 are equalized at the normal operating intermediate frequency. 1 .The operator, in tuning the receiver by knob 18, turns the knob until he sensesit turningunder influence of the clutch 83. He then releases the knob and knows that the receiver is in tune to a carrier wave. Thus he utilizes his sense of touch rather than his aural, or visual sense for his perception of accuratetuning of the receiver.

From the foregoing description it will be apparent that I have provided a tuning arrange ment whereby the system to be tuned is adjusted automatically to resonate exactly at a selected carrier frequency in response to-rough manual adjustment of the system. By displacing: the natural frequencies of the two resonating circuits 88 and 89 only a small amount as,- for example,

two kilocycles, on either side of the operating containers also prevent stray fields from affecting the circuits 88 and 89.

It will,of course, be understood that the automatic tuning arrangement described will be ineffective until the system is manually tuned so that the intermediate frequency lies within the resonant frequency range of the two circuits 88 and 89. Thus, in the specific installation described wherein the two circuits were resonant at 177 and 173 kilocycles respectively and the operating intermediate-frequency was 1'75 kilocycles, the automatic operation was initiated only when the intermediate frequency was within the range extending from a frequency slightly below'173 kilocycles to'a frequency slightly above 177 kilocycles. The initiation of the automatic tuning is, of course, determined in large part by the sharpness with which the two resonant circuits 88 and 89, coupled to the intermediate frequency channel, are tuned to the two selected frequencies above and below the operating intermediate frequency.

In order to secure the best possible operation of my automatic tuning arrangement the resonant frequencies of the two circuits 88 and 89 should be displaced on either side of the operating intermediate frequency by an amount such that the resonance curves of the two circuits intersect on the steep portions of the curves. It has been found that with the resonant frequencies of the circuits 88 and 89 spaced two kilocycles on either side of the operating intermediate frequency and with the circuits tuned sharply to resonance at these frequencies, the resonance curves will intersect at a point on the steep portions thereof in the desired manner. The frequency corresponding to the intersection point of the two resonance curves, of course represents the condition of equal currents in the output circuits of the rectifier tubes 98 and-9| and also the condition of equal currents in the field windings 86 and 81. The point of intersection further represents the operating intermediate frequency to which it is desired that the system be tuned. By tuning the circuits 88 and 89 so that their resonance curves intersect on the steep portions thereof, the dif- 15 ference in the magnitude of the currents in the areasrzs two field windings 86 and 81 is maintained at a substantial value during the automatic tuning operation and until the intersection point correspendingv to exact tuning of the system to the carrier frequency is reached. The considerable difference in magnitude in the currents in the field winding, of course, means that the torque developed by the motor 8| is suflicient to continue the automatic tuning until the system is tuned to exactresonance at the selected carrier frequency. This is an important advantage of my automatic tuning device control circuit since in the absence of a considerable difference between the currents in the two windings 86 and 81 the torque developed by the motor 8| might be insuflicient to effect the desired tuning of the system to exact resonance at the carrier frequency, due to the mechanical inertia and losses in the moving parts of the tuning device.

It will be'readily understood that my novel tuning arrangement is particularly adaptable for use in receiving systems equipped with automatic volume control circuits. It is well known that such circuits tend to maintain a constant volume output regardless of the departure of the system tuning from exact resonance with the selected received carrier. This, to some extent, increases the difficulty of tuning the system since the human ear is comparatively insensitive to changes other than changes in intensity. By

use of my automatic accurate tuning arrangement no skill on the part of an operator or nice aural discrimination is necessary to obtain exact resonant tuning with the selected received carrier. In addition, since the magnitude of the intermediate frequency output is maintained substantially constant by the automatic volume control network, the controlling action of the two resonant circuits 86 and 81 is unaffected by the intensity level of the received carrier. This, of course,"means that the tuning arrangement is positive and accurate in operation under all receiving conditions.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications in the circuits may be made, and I contemplate by the appended claims to cover allsuch modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination, in a superheterodyne receiver, of an input circuit tunable over a range of desired frequencies, a local oscillator tunable over a range of desiredfrequencies, means to tune said input circuit and said local oscillator through said ranges and to maintain the frequency at which the input circuit is tuned different from the frequency to which the local oscillator is tuned by a fixed amount, a detector having a tuned output circuit, said detector being coupled to said input circuit and'to said local oscillator to produce a frequency in said tuned output circuit equal to the difference in frequency between oscillations produced by said local oscillator and oscillations received in said input circuit, and means responsive to oscillations in said tuned output circuit of frequency different from that to which said output circuit is tuned to vary the tuning of both said input circuit and said local oscillator until saidfrequency in said output circuit is substantially the frequency to which said output circuit is tuned while maintaining the frequency to which the input circuit is tuned different from said oscillator frequency by said fixed amount.

2. The combination, in a superheterodyne receiver, of an input circuit tunable over a desired range, a local oscillator circuit tunable over a desired range, means to vary the tuning of said circuits manually through said ranges, and means responsive to the tuning of said circuits to approximate resonance at respective frequencies in said ranges suitable for reception of a desired carrier frequency to increase the accuracy of tuning of both of said circuits at said respective frequencies.

3. The combination, in a superheterodyne receiver, of an input circuit tunable over a desired range, a local oscillator circuit tunable over a desired range, means to vary the tuning of said circuits manually through said ranges, until a desired signal is received, and means responsive to reception of said signal to increase the accuracy of tuning of said circuits to proper frequencies in the respective ranges for most efficient reception of said signal.

4. The combination, in a receiver, of an input circuit, a local oscillator circuit, tuning elements in said circuits arranged for unicontrol by a manually operated member and means responsive to operation of said member approximately to tune said circuits for reception of a desired signal automatically to operate said member to tune said circuits more accurately for reception of said signals.

5. The combination, in a receiver, of an input circuit, a local oscillator circuit, means to tune the receiver to a desired signal, said means including an element in each of said circuits arranged for unicontrol and manually rotatable to effect said tuning, a clutch, and means responsive to rotation of said elements to a position where the receiver is roughly tuned to a desired signal to operate said rotatable element through said clutch to a position more accurately to tune said receiver to receive said signal.

6. In combination, a high frequency receiver, a tuning element for said receiver having a handle adapted to be grasped by the operator, and means responsive to operation of said tuning element to a position roughly tuning said receiver to a carrier wave automatically to increase the accuracy of the tuning of said receiver, and means to indicate to said operator through said handle that said tuning operation is completed.

'7. In combination, a high frequency receiver, a tuning element for said receiver having a handle adapted to be grasped by the operator, and means responsive to operation of said tuning element to a position such that said receiver is approximately tuned to a carrier wave automatically to tune said receiver more accurately to said carrier wave and to actuate said handle thereby to indicate to said operator that said receiver is tuned to said carrier wave.

8. In a radio receiving system including a signal input tunable circuit, means for impressing on said input circuit a plurality of incoming carrier waves spaced in adjacent transmission channels of a frequency spectrum, manually operable means for tuning said input circuit to approximate resonance with a selected one of said incoming carrier waves impressed on said input circuit, and signal controlled means responsive to operation of said manually operable means to tune said circuit to approximate resonance with said selected incoming carrier for automatically tuning said input circuit to exact resonance with said selected carrier wave subsequent toisaid manual tuning, V

9. In a radio receiving system of the type including an input circuit, a local oscillator and an intermediate frequency transmission channel, means for impressing on said input circuit a plurality of incoming carrier waves spaced in-adjacent transmission channels of a frequency spectrum, manually operable means for tuning said input circuit to approximate resonance with a selected one of said incoming carrier waves and for tuning said local oscillator to a frequency different by a fixed amount from the frequency to which said input circuit is tuned, and means coupled to said intermediate frequency transmission channel and responsive to operation of said manually operable means for automatically tuning said input circuit to exact resonance with said selected carrier wave and said local oscillator to said frequency different by said fixed amount from the frequency to which said input circuit is tuned.

10. In a superheterodyne receiving system, a signal input tunable circuit, means for impressing on said input circuit a plurality of incoming carrier waves spaced in adjacent transmission channels of a frequency spectrum, means for tuning said circuit to resonance with a selected one of said incoming signal carrier waves, manually operable means for actuating said tuning means to effect an approximate tuning of said circuit to resonance with said selected carrier wave, auxiliary means for actuating said tuning means, an intermediate frequency amplifier having an output circuit, and means including a pair of resonant circuits coupled to said output circuit and tuned respectively above and below the operating intermediate frequency for controlling the energization of said auxiliary means in response to operation of said manually operable means thereby to automatically control the adjustment of said tunable circuit to exact resonance with said selected carrier Wave.

11. In a radio receiving system, a tunable signal input circuit, a tunable local oscillator circuit, means for impressing on said input circuit a plurality of incoming carrier waves spaced in adjacent transmission channels of a frequency spectrum, means for simultaneously tuning both of said circuits, said means tuning said input circuit to approximate resonance with a selected one of said incoming signal carrier waves, means for automatically actuating said tuning means to tune both of said circuits, said last named means tuning said input circuit to exact resonance with said selected incoming signal carrier wave, an intermediate frequency amplifier having an output circuit, and means including a pair of resonant circuits coupled to said output circuit and tuned respectively above and below the operating intermediate frequency for controlling the energization of said automatic actuating means.

12. In a radio receiving system, a tunable signal input circuit, means for impressing on said input circuit a plurality of incoming carrier waves spaced in adjacent transmission channels of a frequency spectrum, a local oscillator circuit, an intermediate frequency amplifier having an input circuit tuned to a predetermined operating frequency and an output circuit, means for simultaneously tuning said oscillator and signal circuits, manually operable means for actuating said tuning means to effect an approximate tuning of said input circuit to resoincluding a. pair of resonant circuits coupled to said output circuit and tuned respectively above and below said operating frequency, means for rectifying the current flowing in said resonant circuits, and a network energized in accordance 5 with the diflerential of said rectified currents.

CHAUNCEY G. SUITS. 

